1. Field of the Invention
The present invention relates to a current-controlled semiconductor device (power device) such as a bipolar transistor or a bipolar static induction transistor.
2. Description of the Related Art
FIG. 5 shows a cross sectional structure of a bipolar transistor with a guard ring structure achieved in the related art. As shown in FIG. 5, an n-type collector region 52 is formed on an n+ substrate region 51, with a p-type base region 58 formed in the upper collector region 52. In addition, an n+ type emitter region 53 is formed in the upper base region 58.
At the upper surface of the collector region 52, an insulating film layer 60 is formed. A collector electrode 61 achieves ohmic contact with the substrate region 51, whereas an emitter electrode 63 achieves ohmic contact with the emitter region 53 via an emitter contact hole provided at the insulating film layer 60. Base electrodes 68 achieve ohmic contact with the base region 58 via base contact holes provided at the insulating film layer 60.
A p-type guard ring region 70 is formed in a ring shape so as to enclose the base region 58 and the emitter region 53 and is not connected to any of the electrodes. It is to be noted that while FIG. 5 shows a single guard ring region 70, the quantity of guard ring regions 70 that should be provided is determined in correspondence to the required withstanding voltage and the like.
It is to be noted that while FIG. 5 shows a single unit bipolar transistor, a plurality of bipolar transistor unit structures are formed in conformance to the required current capacity.
The operation of the bipolar transistor (device) achieved in the related art shown in FIG. 5 is now explained. This bipolar transistor is utilized by applying, for instance, a ground potential (0 V) to the emitter electrode 63 and applying a positive potential to the collector electrode 61. In addition, the base electrode 68 and the emitter electrode 63 are connected with a base drive circuit provided as an external circuit so as to allow the bipolar transistor to be driven as a switching device. ON/OFF control on the bipolar transistor is achieved in response to a signal provided by the base drive circuit.
When the ground potential or a negative potential is applied to the base electrode 68 to set the bipolar transistor in an OFF state, a reverse bias is applied between the p-type base region 58 and the n-type collector region 52, resulting in an extension of a depletion layer. If the potential applied to the collector electrode 61 is raised at this time, an avalanche breakdown occurs between the base region 58 and the collector region 52.
If an avalanche breakdown occurs between the base region 58 and the collector region 52 in a current-controlled semiconductor device such as the bipolar transistor in the related art, the avalanche breakdown current is allowed to flow to the base drive circuit. Accordingly, it is necessary to take measures such as the use of a device with a large current capacity to constitute the base drive circuit, in order to ensure that the base drive circuit is not damaged by the avalanche breakdown current.
An object of the present invention is to provide a semiconductor device achieving a structure that prevents the base drive circuit from being damaged by the avalanche breakdown current.
A semiconductor device according to the present invention, which is a current-driven semiconductor device having a semiconductor substrate of one conduction type constituting a collector region, an emitter region of a conduction type which is the same as the conduction type at the collector region and a base region of a conduction type opposite from the conduction type at the collector region both formed in contact with one main surface of the collector region, and includes a cathode region of a conduction type opposite from that at the collector region, which is in contact with the main surface but is not in contact with either the emitter region or the base region, with the cathode region achieving a potential equal to the potential at the emitter region and formed in a ring shape so as to enclose the emitter region and the base region.
A semiconductor device according to the present invention, which is a current-driven semiconductor device having a semiconductor substrate of one conduction type constituting a collector region, an emitter region of a conduction type which is the same as the conduction type at the collector region and a base region of a conduction type opposite from the conduction type at the collector region both formed in contact with one main surface of the collector region includes first trenches provided so as to lie in contact with the main surface and to sandwich the emitter region, first fixed-potential insulated electrodes provided inside the first trenches, insulated from the collector region by a first insulating film and achieving a potential sustained at a level equal to the potential at the emitter region, a channel region that constitutes a portion of the collector region that is in contact with the emitter region and is sandwiched by the first fixed-potential insulated electrodes and a cathode region of the conduction type opposite from that at the collector region, which is in contact with the main surface but is not in contact with either the emitter region or the base region. In this semiconductor device, the base region is in contact with the main surface, the first insulating film and the collector region but is not in contact with the emitter region, the first fixed-potential insulated electrodes are constituted of a conductive material achieving a work function which allows a depletion layer to be formed at the adjacent collector region via the first insulating film, the distance between the bottoms of the first trenches to the emitter region at the channel region, i.e., the channel length, is at least twice the distance between the sidewalls of the first trenches facing opposite each other at the channel region, i.e., the channel thickness, the potential at the cathode region is sustained at a level equal to the potential at the emitter region and the cathode region is formed in a ring shape so as to enclose the emitter region, the first fixed-potential insulated electrodes and the base region.